Descriptions

Uptake and cycling of nitrogen (N) in mature trees are not well understood.
Fertilizer-N uptake, partitioning, and use-efficiency were studied in standard strain
'Topred Delicious' and spur strain 'Redspur Delicious' mature apple trees {Malus
domestica Borck) on 'Malling 7A' (M.7A) rootstocks. The treatments consisted of a
ground application in spring, a preharvest ground application in August, a foliar spray
in fall, or a combination of each of the last two treatments with the first. When soil-applied
in spring, labeled N (¹⁵N) was allocated preferentially to above ground tissues
and to a lesser extent to roots of both strains. The amount of newly absorbed soil-N
allocated to above ground tissues decreased as the season progressed, little ¹⁵N from preharvest
ground applications reached the leaves, fruit, buds, or branches, while roots were
heavily labeled.
Total fertilizer-N recovery in the trees was similar regardless of the time of
fertilizer application. However, losses of ¹⁵N to fruit removal, leaf fall, and pruning
were most severe when N was applied in spring and minimal for the pre-harvest timing
compared to all others. About a third of the variability in recovery was due to variation
in tree size. When recoveries were adjusted to account for size differences, spur-type
trees tended to be more efficient at utilizing fertilizer-N.
When ¹⁵N-urea was applied in April to young apple leaves, the label was not
exported. Labeled N from fall urea sprays was exported from the leaves to the buds, but
was restricted to the treated spurs and branches. Foliar urea sprays immediately after
harvest contributed more N to the buds than later applications.
Mobilization of ¹⁵N from storage in various tree parts was assessed. In
moderately vigorous trees, N stored in aerial parts of the tree was mobilized first,
followed by simultaneous mobilization of root and soil N. Utilization of root reserves
depended on the N status of the tree. When the buds were low in N but the roots had
adequate N reserves, root to shoot N transport started early in winter.